A utility knife is a tool for cutting hard materials such as leather, rubber, etc. It comprises a changeable knife blade and a knife body. Among the utility knives currently on the market, there are two main types of mechanisms for holding the knife blade onto the knife body. One type is bolt fastening. Another type is laterally pressuring. These two types of mechanisms have common problems of inconvenient to install the blade and difficult to tighten the blade. For the bolt fastening type, changing the knife blade is inconvenient, and a gap between the bolt and the blade would make the blade slightly movable, therefore affecting the precision of the cutting. Meanwhile, the size of the bolt cannot be too small. For the laterally pressuring type (i.e. the blade is installed by opening one side panel of a two-panel body, placing the blade inside, and closing the side panel using a U-shaped clamp to hold the two panels and blade together), has the problem of using only one edge of the body panels for applying the pressure to secure the blade. The pressure to secure the blade, coming from the U-shaped clamp, may not be strong enough, and the pressure is conducted by the body panels to the blade, not directly applied on the blade. When in operation, a force applied on the object being cut generates a reacting force on the knife. This reacting force may affect the stability of the blade.